An organic EL element constituting a display panel using an organic material is generally formed from a glass substrate as a display surface, an anode as a transparent electrode, plural layers of organic materials containing an organic emissive layer and a cathode consisting of a metal electrode, each prepared in thin film and stacked one upon another. The layers of organic materials contain layers capable of transporting holes such as a hole injecting layer and a hole transporting layer and layers capable of transporting electrons such as an electron transporting layer and an electron injecting layer in addition to an organic emissive layer and organic EL elements comprising such layers of organic materials have also been proposed. An inorganic compound may as well constitute an electron injecting layer.
When an electrical field is applied to an organic EL element comprising an organic emissive layer and an electron or hole transporting layer, holes are injected from the anode and electrons from the cathode. These electrons and holes recombine together in the organic emissive layer to form excitons which return to the ground state with emission of light and an organic EL element utilizes this emission of light. The emissive layer is often doped with a colorant as a guest material to raise the luminous efficiency or to operate the element stably.
In recent years, the use of phosphorescent materials besides fluorescent materials in the emissive layer has been proposed. In the emissive layer of an organic EL element, the probability of generating singlet excitons and triplet excitons after recombination of electrons and holes is thought to be 1:3 and an element utilizing phosphorescence by triplet excitons as well is thought to attain luminous efficiency three to four times greater than that of an element utilizing fluorescence by singlet exictons.
On the other hand, for the purposes of reducing the power requirement and improving the luminous efficiency and operating stability of an organic EL element, a proposal has been made to provide a hole blocking layer between the organic emissive layer and the cathode to restrict the migration of holes from the organic emissive layer. This hole blocking layer accumulates holes efficiently inside the emissive layer thereby improving the probability of recombination of holes with electrons and raising the luminous efficiency. Phenanthroline derivatives and triazole derivatives have been reported to be effective as hole-blocking materials.    Patent literature 1: JP4-206685A    Patent literature 2: JP2001-237079A    Patent literature 3: JP2001-284056A
It is reported in JP04-206685A that a complex of aluminum with a hydroxyquinoline compound and a phenolic compound (hereinafter referred to as A1Q2OR) is useful as an organic EL material emitting blue light. This A1Q2OR has a structure in which one aluminum atom is complexed with two molecules of 8-hydroxyquinoline compounds and one molecule of a phenolic compound. In an example disclosed in JP04-206685A, A1Q2OR is incorporated in the electron transporting layer to emit light.
A phosphorescent or fluorescent organic EL element reported in JP2001-237079A has A1Q2OR in the hole blocking layer. Further, a phosphorescent organic EL element reported in JP2002-284056A has a hole blocking layer comprising A1Q2OR between the emissive layer containing a phosphorescent material and the electron transporting layer.
A compound cited in JP2001-237079A and JP2001-284056A for A1Q2OR is (1,1′-biphenyl)-4-olato)bis(2-methyl-8-quinolinolato-N1,08)aluminum (hereinafter referred to as BA1q) obtained from 2-methyl-8-hydroxyquinoline as a hydroxyquinoline compound and 4-phenyphenol as a phenolic compound. Although BA1q shows excellent durability, it has a shortcoming of inferior hole-blocking ability as its ionization potential (Ip) is not sufficiently large. Consequently, in the cases where BA1q is used in the hole blocking layer and tris(8-hydroxyquinolinato-N1,08)aluminum (hereinafter referred to as Alq3) is used in the electron transporting layer, it is the electron transporting layer that emits light. In an organic EL element utilizing red phosphorescence, the emission of light (green) by Alq3 leads to degradation of chromaticity. Now, it has been found in some cases that, in an organic EL element whose emissive layer comprises a phosphorescent material as a guest material, the use of A1Q2OR as a host material is capable of attaining a long operating life while maintaining good luminous characteristics.
However, the life of an element using A1Q2OR disperses a great deal and this has become a large obstacle in putting an element of this kind into practical use. The cause of this dispersion of life has not been elucidated at all. As a result, there has not been a clue for setting up guidelines not only for the control of materials essential for the manufacture of highly reliable commercial elements but also for improvement of life.